1. Field of the Invention
This invention generally relates to duplexers, and more particularly, to a duplexer having a resonance circuit connected to an antenna terminal.
2. Description of the Related Art
A filter for high-frequency communications is available. Generally, such a filter is composed of multiple resonators and is capable of allowing only electric signals in a particular frequency range to pass. The resonators may use a piezoelectric material and utilize a surface acoustic wave (SAW) or a bulk acoustic wave (BAW). The filter devices that utilize SAW or BAW have a compact outer size and a sharp roll-off characteristic, as compared to dielectric filters and ceramic filters. Thus, the SAW or BAW filter devices are suitable for mobile communication devices of portable telephones required to have a compact size and a small ratio of the signal bandwidth to the center frequency. A duplexer is a device to which the SAW or BAW filters are applied. The duplexer is employed in radio frequency communication equipment capable of transmitting and receiving signals at different frequencies. The SAW or BAW filter may be formed by arranging resonators in a ladder configuration. The insertion loss and out-of-band rejection of the ladder type filter may be easily adjusted by simply changing the number of stages of the ladder configuration and the capacitance ratio between the series and parallel resonators. Further, the design sequence is simple. For the above-mentioned reasons, the ladder type filters are widely used.
Japanese Patent Application Publication No. 2003-332885 discloses a duplexer composed of first and second filters and a matching circuit, in which inductors are connected to series resonators of the first and second filters (see FIG. 1). Japanese Patent Application Publication No. 10-313229 discloses a duplexer composed of first and second filters and a matching circuit in which an inductor that forms the matching circuit is connected between an antenna terminal and ground.
The duplexer used in mobile communications or high frequency wireless communications should be designed to consider IMD (Inter Modulation Distortion) and CMD (Cross Modulation Distortion). These adverse factors arise from the non-linearity of the structure and material of the resonators. When two or more signals of different frequencies (which may, for example, a transmission signal and a disturbance wave) are applied to the duplexer, the non-linearity of the duplexer may result in a signal of a reception frequency capable of passing through the reception system of the duplexer. The signal passing through the reception system is noise.